Rejig serial periph

README.md

@@ -30,20 +30,19 @@ these headers.
 |--------|--------|----------|--------------|
 | TXBUF  | 0      | 32       | write        |
 | RXBUF  | 32     | 32       | read         |
-| TXDATA | 64     | 1        | atomic write |
-| TXHEAD | 65     | 1        | atomic read  |
-| TXTAIL | 66     | 1        | atomic write |
-| RXDATA | 67     | 1        | atomic read  |
-| RXHEAD | 68     | 1        | atomic write |
-| RXTAIL | 69     | 1        | atomic read  |
+| TXHEAD | 64     | 1        | atomic read  |
+| TXTAIL | 65     | 1        | atomic write |
+| RXHEAD | 66     | 1        | atomic write |
+| RXTAIL | 67     | 1        | atomic read  |
 
-For both sending (`TX`) and receiving (`RX`), there are four
-registers: `xBUF`, `xDATA`, `xHEAD` and `xTAIL`:
+For both sending (`TX`) and receiving (`RX`), there are three
+registers: `xBUF`, `xHEAD` and `xTAIL`:
 
 - `xBUF` registers are 32-byte FIFO ring buffers used for data
-- The `xDATA` registers indicate whether data is available (0 for
-  data, FFh for no data)
-- The `xHEAD` and `xTAIL` registers specify the start and end of
-  data in the FIFO, `xHEAD` being the offset of the first byte of
-  data, and `xTAIL` being the offset of the first byte after the
-  data.
+- The `xHEAD` and `xTAIL` registers specify the start and end of data
+  in the ring buffer, `xHEAD` being the offset of the first byte of
+  data, and `xTAIL` being the offset of the first byte *after* the data.
+
+In order to be distinguishable from the empty state, the ring buffers
+must never be completely full -- there must always be *at least one*
+unoccupied byte between the tail and the head.

emu.js

@@ -1,15 +1,13 @@
 const TXBUF = 0;
 const RXBUF = 32;
-const TXDATA = 64;
-const TXHEAD = 65;
-const TXTAIL = 66;
-const RXDATA = 67;
-const RXHEAD = 68;
-const RXTAIL = 69;
+const TXHEAD = 64;
+const TXTAIL = 65;
+const RXHEAD = 66;
+const RXTAIL = 67;
 
 const TXBUF_SIZE = 32;
 const RXBUF_SIZE = 32;
-const PERIPHS_SIZE = 70;
+const PERIPHS_SIZE = 68;
 
 const POLL_INTERVAL_MS = 20;
 
@@ -36,29 +34,21 @@ class Emulator {
 	}
 
 	poll() {
-		const txdata = Atomics.load(this.mem_u8, TXDATA);
-		if (txdata !== 0)
-			this.handle_txdata();
+		const txhead = Atomics.load(this.mem_u8, TXHEAD);
+		const txtail = Atomics.load(this.mem_u8, TXTAIL);
+		if (txhead !== txtail)
+			this.handle_txdata(txhead, txtail);
 	}
 
-	handle_txdata() {
-		const head = Atomics.load(this.mem_u8, TXHEAD);
-		const tail = Atomics.load(this.mem_u8, TXTAIL);
-
+	handle_txdata(head, tail) {
 		const data = [];
 		let i = head;
 		do {
 			data.push(this.mem_u8[TXBUF + i]);
 			i = (i + 1) % TXBUF_SIZE;
 		} while (i !== tail);
-
 		Atomics.store(this.mem_u8, TXHEAD, tail);
 
-		// More data could have been added -- only clear TXDATA if
-		// tail is unchanged.
-		if (tail === Atomics.load(this.mem_u8, TXTAIL))
-			Atomics.store(this.mem_u8, TXDATA, 0);
-
 		const str = this.decoder.decode(new Uint8Array(data));
 		this.output.innerText += str;
 	}